Small wind turbines (SWTs) are an increasingly popular means to generate renewable energy worldwide. Flexibility in size and design allow SWTs to be installed in a much wider range of settings compared to large wind turbines. While large wind turbines can cause substantial mortality of birds and bats, the extent and correlates of such collision mortality at SWTs have not been quantified. Thus, siting decisions for SWTs are currently made with considerable uncertainty of their impact on wildlife. We combined field data and questionnaire surveys of SWT owners to assess the range and correlates of bird and bat mortality at SWT sites (mean hub height 10.2 m [range 4.0-26.4 m], mean rotor diameter 4.0 m [range 0.9-15.0 m], for both free-standing and building mounted turbines). During 171 carcass searches at 21 UK SWT sites we did not find any collision casualties. Thirty-one (14.6 %) of 212 SWT owners reported bird casualties of at least 12 species groups and 3 (1.4 %) reported bat casualties (unidentified species). Based on the questionnaire returns and using a model that accounts for detectability of casualties (through variation in visit frequency, searcher efficiency and average levels of scavenger removal) we estimated that between 0.079 and 0.278 birds, and between 0.008 and 0.169 bats may be killed turbine-1 year-1, equating to 1,567-5,510 birds and 161-3,363 bats year-1 in the UK based on recent estimates of numbers of units installed. Sites with higher levels of bird activity tended to be more likely to report bird casualties. Systematically derived likely ranges of mortality as provided here are urgently needed to inform future SWT planning policy.